1. Field of the Invention
The present invention relates to a method and apparatus for automatic dilution of a starting concentrated fluid suspension of particles for the purpose of optimizing a measurement of the particle size distribution, where the measurement technique chosen is sensitive to individual particles in the suspension over some appropriate range of particle size.
2. Description of the Prior Art
There are many intermediate process materials and final products of technical and/or commercial significance which exist either as a relatively high concentration of solid particles or liquid droplets suspended in a fluid, or as powders, which can be suspended in an appropriate liquid. The physical and/or chemical characteristics of these particle suspensions or dispersions (referred to herein as "sample suspensions"), or of the final products derived from them, often depend critically on the particle size distribution (PSD). Hence, it has become increasingly important to determine the PSD of these particle suspensions with high accuracy, resolution and reproducibility, often in the early stages of production. The solid particles or liquid droplets (in the case of oil-in-water or water-in-oil emulsions)--known as the "dispersed phase"--are suspended in an appropriate fluid, consisting of a simple liquid (e.g. water or an organic solvent) or mixture of liquids, perhaps containing one or more additives of various kinds (e.g. surfactants)--known as the "continuous phase". Typically, the starting particle suspensions to be analyzed contain a relatively high concentration of the dispersed phase--usually exceeding 10% by weight or volume of the overall fluid suspension, and sometimes reaching 40-50%.
Most methods of particle size analysis (PSA) require that the sample presented to the analyzer be much less concentrated than that which is typically available from an intermediate process stream or final product. This requirement that the starting sample be diluted, sometimes very substantially, prior to determination of its PSD is particularly critical for methods known as single-particle sensing (SPS) techniques. Because these methods demand relatively low concentrations of suspended particles in order to produce PSD results of high accuracy and minimal distortion, they are effective in providing a motivation for the present invention.
The well-known method of single-particle optical sensing (SPOS), described extensively elsewhere, is one particular kind of SPS method. It utilizes the principle of either light extinction or scattering to determine the mean diameters of suspended particles as they pass individually through a very small sensing "zone" or "view volume". In the recent invention of Wells et al (pending U.S. patent application Ser. No. 08/625,540, filed May 28, 1996, now U.S. Pat. No. 5,835,211), the physical principle of light scattering is combined with that of light extinction in order to increase the sensitivity and dynamic size range of the sensor. The resulting hybrid design thus extends the applicability of the SPOS method to particles smaller than those which would be detectable using only the light extinction technique, while preserving the large size range offered by the latter. This improvement increases the usefulness and overall effectiveness of the SPOS method.
There is another well-known SPS method for particle size analysis which is based on a different physical principle--the "electro-zone", or "Coulter counter", method. In this well-known technique, one monitors the conductivity between two volumes of partially conducting liquid, one of which contains the suspended sample particles at relatively low concentration, connected together by a small pore. The conductivity decreases momentarily whenever a particle passes through the connecting pore, caused by a pressure differential applied between the two liquid volumes. The magnitude of the conductivity decrease is proportional to the volume of the particle which momentarily interrupts the current flow through the pore. This represents another viable SPS method which can be used in conjunction with an autodilution system based on the present invention.
Regardless of the specific SPS method which is used to perform a PSA measurement, a concentrated particle suspension typically requires extensive predilution in order to ensure accurate PSD results with minimum distortion and artifacts. Specifically, this step is needed to ensure that the particles pass through the active sensing volume, or zone, one at a time, thereby avoiding all but occasional "coincidences" and consequent distortions of the output signal pulses and resulting PSD. For the purpose of explaining the underlying principles of this invention, it is convenient to focus exclusively on the use of an SPOS-type sensor in conjunction with the automatic dilution method and apparatus to be described. However, no loss of generality is thereby intended or implied. Rather, it is implicitly assumed that other methods of particle size analysis, including but not limited to other SPS methods, such as the electro-zone technique, may be used in place of the SPOS method in conjunction with this invention. Examples include "ensemble-type" methods for PSA, which respond to many particles at the same time, such as dynamic light scattering (DLS) and Fraunhofer diffraction (FD), also known as "laser" diffraction (LD). These techniques also usually require substantial dilution of concentrated particle suspensions, depending on their composition and particle size range (i.e. PSD). However, the extent of dilution typically required for these ensemble techniques is often considerably smaller than that needed for SPS techniques, such as SPOS and electro-zone sensing.
Therefore, PSA measurements, including those performed using the SPOS method, generally require substantial dilution of the original concentrated particle suspension, using an appropriate fluid, or mixture of fluids, as a diluent. This is especially required for automatic "online" particle size analysis of process suspensions in a production environment. In this case a dilution system should be able to accommodate samples with greatly differing PSDs, without requiring knowledge of their concentration, composition or PSD characteristics. The extent of dilution of the starting sample should be arrived at quickly and be optimal for the particle size analysis method being utilized--e.g. SPOS.
There are numerous applications for PSA in which the concentration of the starting particle suspension changes relatively little from one sample to the next. In such cases, it may be sufficient to dilute the starting particle suspension using a fixed, predetermined dilution factor, DF. This fixed DF value can be determined ahead of time, in trial-and-error fashion, for each kind of sample or application. A variety of prior art methods and devices exist for diluting a fluid sample with a predetermined dilution factor. By definition, these prior art methods and devices cannot provide for variable dilution when such would be more useful than the fixed, predetermined dilution which they provide, and therefore they are of limited utility.
Examples of prior art methods and/or devices which can be used to provide a fixed, predetermined dilution of fluid samples are described in: Cruzan, U.S. Pat. No. 4,036,062, Roof et al, U.S. Pat. No. 4,036,063, and Roof, U.S. Pat. No. 4,070,913. All of these patents describe means for diluting a fluid sample with a diluent fluid in which each of the fluids is initially contained in separate conduits. At the start of the dilution process the two conduits are connected together to permit closed-loop circulation and mixing of the two fluids. The extent of dilution--i.e. the value of the dilution factor, DF--is determined ahead of time by preselecting the volumetric relationship (relative volumes) of the two conduits.
This traditional approach to diluting a starting concentrated particle suspension is, by definition, inflexible and therefore quite limited. It is also potentially inaccurate when relatively large dilution factors are required. In such situations it may be problematic to inject, or meter out accurately, a very small volume of starting particle suspension into a much larger volume of diluent fluid. This limitation can in general be overcome by performing multiple dilutions in succession, where each dilution step has a fixed, relatively small DF value, able in principle to be accurately controlled. The final dilution factor is then equal to the product of all of the individual, intermediate DF values. However, the apparatus needed to implement this multiple-dilution approach is necessarily more complex and difficult to maintain than a simple, single-stage dilution device.
There exist prior art dilution systems which introduce both the starting concentrated fluid sample and diluent fluid into a mixing chamber on a continuous basis. The rates of flow of each of these input components can be adjusted to fixed, known values so as to yield a final diluted fluid sample having a known dilution factor DF equal to the ratio of the total fluid flow rate (diluent plus starting concentrated sample) to that of the starting concentrated sample alone. The final diluted sample suspension is typically extracted from the mixing chamber at a steady flow rate. Such a dilution system, having a known, but adjustable, dilution factor, is described in the invention of Mowery, Jr., U.S. Pat. No. 4,095,472. In this patent a fixed dilution factor is established at the start of the dilution process. In principle, the dilution factor can range from a relatively low value to a very high one. This method will be described in greater detail subsequently. The invention of Culbertson, U.S. Pat. No. 3,805,831, describes an apparatus for continuously and proportionately mixing one fluid stream, containing concentrated solute, with another, acting as the diluent. The final solute concentration which emerges in the resulting fluid stream is determined by the composition of each individual fluid stream and their relative rates of flow.
Automatic dilution systems which rely on the principle of negative feedback have also been described. In these systems, one or both of the flow rates of the starting, concentrated solute sample and the diluent fluid which enter the mixing chamber are continuously adjusted so as to yield an approximately unchanging solute concentration in the fluid mixture which exits the chamber. Mechanisms for adjusting the flow rate(s) have been proposed which respond to a measurement of the turbidity (i.e. optical density, or absorbence, at a particular wavelength or range of wavelengths) or scattered light intensity (over a particular range of angles) or diffracted light intensity obtained from the diluted fluid mixture residing in, or exiting from, the mixing chamber. A dilution system which responds to one of these measurements is implied by the invention of Pardikes, U.S. Pat. No. 4,279,759, which describes the use of optical sensing devices to measure the presence of a treatment chemical in a liquid process stream. Using negative feedback, this invention controls the rate of introduction of the treatment chemical into the continuously flowing stream so as to establish a relatively fixed, but adjustable, concentration of the chemical in the stream. By extension, sensing techniques based on light scattering and/or diffraction, as described in the inventions of Moreaud et al, U.S. Pat. No. 4,348,112, Tsuji et al, U.S. Pat. No. 4,408,880, and Brenholdt, U.S. Pat. No. 4,507,556, can be used to adjust the flow rate(s) of one or both of the starting concentrated fluid sample and diluent fluid entering a mixing chamber for the purpose of holding relatively constant the solute concentration in the resulting diluted sample fluid.
Finally, the method and apparatus described in the invention of Nicoli et al, U.S. Pat. No. 4,794,806, is able to dilute a starting concentrated particle suspension, using the principle of (approximate) exponential dilution of particles suspended in fluid in a mixing chamber of fixed volume due to the continuous addition of a diluent fluid at a known flow rate. Unlike the methods and devices described above, based on continuous mixing of both the concentrated sample fluid and the diluent fluid, the method described by Nicoli et al is based on injection of a fixed amount of sample into the mixing chamber. Hence, the amount of diluted sample material (i.e. suspended particles) available for the PSA measurement process at the output of the mixing chamber is limited by the amount which was originally injected into the chamber at the start of the automatic dilution process.